1. Field of the Invention
The invention relates to the field of the growth of bulk monocrystalline BeO from flux melts.
2. Prior Art
Prior art bulk monocrystalline BeO has been grown from flux melts of lithium or potassium molybdate including growth modifying impurities present in ranges from nil to 1% by weight of the molybdate flux. Thermal gradient transport of BeO from a polycrystalline BeO source to monocrystalline seed plates has been utilized. The prior art procedure for providing seed plates has been to slice, trim and drill monocrystalline BeO seed plates and support them by platinum wires passing through a hole drilled in the seed plate. These seed plates have been carefully selected to be substantially free of such defects as inclusions, dislocations and twin boundaries. These seed plates are prepared with a (1010) crystal face as a major growth face because additional monocrystalline BeO grows rapidly on this face. However, as growth proceeds on the (1010) faces of the seed plate, they are inherently replaced by faces such as (1011) and vicinal (0001) all of which grow at markedly lower rates than the (1010) faces. Because of this inherent replacement of a fast growing face by slower growing faces, the growth of large crystals in this manner is very time consuming. The (1010) face may grow at a rate of one-half to one millimeter per week whereas the replacement faces grow at much slower rates on the order of 0.001 to 0.01 millimeter per week. The growth of additional material on the seed plates is accomplished in a supersaturated region of the flux. This results in both the growth of additional BeO on the seed plates and the undesired nucleation of additional crystals on the surfaces of the crucible and crystal support rig. These newly nucleated crystals can lead to defects in the seed plates when excessive nucleation takes place and limit the length of growth runs by physical interference with the desired crystals. Excessive nucleation can also cause a reduction in the supersaturation of the melt because of increased deposition area. This reduction in supersaturation reduces the rate of growth of the desired crystal. Consequently, it is necessary to periodically remove the seed crystals from the melt and remove undesired crystals from the crystal support rig and the surfaces of the crucible. The crystals so removed may be returned to the BeO source for use as BeO source materials in subsequent growth runs.